ES2728309T3 - Printed circuit boards - Google Patents

Abstract

A method of making a solder connection to a printed circuit board, in which: a surface of the printed circuit board has a coating comprising one or more halo-hydrocarbon polymers with a thickness of 1 nm to 10 μm ; and the printed circuit has conductive tracks and there is no welding, or essentially no welding, between the coating and the conductive tracks, where said method comprises applying welding and flux to the coating without its prior removal, at a temperature and for a time so that the solder adheres to the conductive tracks and the coating is dispersed and / or absorbed and / or vaporized locally.

Description

DESCRIPTION

Printed circuit boards

The present invention relates to articles such as those comprising printed circuit boards coated with a halo-hydrocarbon polymer.

Printed circuit boards (PCI) are used in the electronics industry to mechanically support and electrically connect electrical and electronic components. A PCI comprises a plate or other substrate made of an insulating material on which conductive tracks, normally made of copper, rest. These conductive tracks function as wires between the electrical components that are then fixed to the board, for example, by welding. A large proportion of PCI are manufactured by depositing or otherwise adhering a layer of copper to the substrate plate and then removing unwanted copper by chemical etching to leave the copper tracks in the required configuration. At this stage, blank PCI can often be stored for varying periods of time, potentially up to several months, before fixing the electronic components to the PCI using a welding method.

The conductive tracks on the printed circuit board can be made of any conductive material. The preferred material for the tracks is copper. Copper is the preferred material for conductive tracks, mainly due to its high electrical conductivity, but unfortunately copper easily oxidizes in the air and leads to a layer of copper oxide, or tarnish, on the metal surface. This oxidation is particularly evident if a long period of time has elapsed between manufacturing the blank PCI and fixing the electrical components. The components are fixed by welding, but the presence of an oxide layer on the copper tracks can reduce the welding efficiency. In particular, dry joints can be formed, which tend to fail during device operation, and weak joints with low mechanical strength. Occasionally, the board may not make full electrical contact. Similar problems arise when the conductive tracks comprise conductive materials other than copper.

To minimize these problems, PCI manufacturers apply a range of coatings or surface finishes to areas where welding will be required. Metals such as tin, silver or a nickel / gold combination are often used. The processes for the application of these finishes are time consuming and require the use of additional metals, with the consequent environmental problems. There are potential health problems associated with some of the processes and materials. Additionally, some of the metals used, such as gold, are expensive. A similar approach involves coating the tracks with a coating comprising organic compounds such as benzimidazoles and metal particles wettable by welding or welding (see, for example, WO 97/39610), thereby avoiding exposure of the tracks to oxidative conditions During welding, the organic layer is simply removed. These organic coatings usually do not survive multiple thermal cycles and have a relatively short storage life before processing.

It is clear that the techniques adopted by manufacturers so far are expensive or time-consuming (imply additional stages in the manufacturing process), or both, and deplete non-renewable resources, including precious metals. There is a need for a more economical and / or higher performance method of preventing oxidation of the conductive tracks before joining the electrical components by welding.

A separate problem is that PCI is often required in devices that are used in very harsh and corrosive environments. Under such conditions, the conductive tracks on the pCi can corrode and lead to a much shorter life of the circuit board than would normally be expected. Such conditions may arise, for example, when a device is used in very humid environments, especially when microscopic droplets of water containing dissolved gases such as sulfur dioxide, hydrogen sulfide, nitrogen dioxide, hydrogen chloride, chlorine and steam water, form a corrosive solution. Additionally, moisture droplets can form a thin film or corrosion deposits between the conductive tracks of the PCI that can potentially cause short circuits. In circumstances where PCI manufacturers anticipate that the devices will be used in hostile conditions, they have tended to coat the assembled PCI with a polymer conformation coating that forms a barrier to the environment. However, such coatings are expensive to apply and require an additional stage in the manufacturing process to apply the coating after the pCi has been assembled and, generally, an additional stage later to remove it. This can also cause problems when reworking a damaged or failed PCI, or during the test to determine its performance and solve a problem. A cheaper and / or high-performance method to environmentally protect the completed PCI would be of great interest to manufacturers.

Another problem that may arise after welding electronic components to a PCI is the formation of metal compound dendrites in the solder joint. These dendrites can cause the failure of the assembled PCI due to short circuits between the contacts. Dendrites are very thin metallic growths along a surface, resulting from electromigration, that form fern-like patterns. The mechanism Growth for dendrites is well known, unlike "tin whiskers," and requires the presence of moisture that generates metal ions that are then redistributed by electromigration in the presence of an electromagnetic field. The coating of the invention protects against dendrite formation by preventing moisture from reaching the surface of the PCI, which is where dendrites normally grow. The coating provides additional protection, since the dendrite materials have a low adhesion to the surface coating, reducing the formation of dendrites between the contacts and the components.

US 2006/0001700 describes a flexible circuit that has corrosion resistance and a method for doing so. WO 97/39610 describes an organic-metallic composite coating for the protection of the copper surface. US 2006/0292354 describes a wiring substrate that has a metal wiring pattern and an organic film containing silane. Deltschew: "Plasma treatment for fluxless soldering " Surface & Coatings Technology Elsevier Switzerland, vol. 142-144, July 2001, page 803-807 describes the use of plasma treatment processes in reflux welding without flux.

US2002 / 0134580 describes a printed circuit board with a layer of plasma polymerized polymer. The present specification provides a printed circuit board to which a welding connection is to be made, where the surface of said printed circuit board has a coating of a composition comprising one or more halo-hydrocarbon polymers, with a thickness of 1 nm to 10 pm, in which there is no welding or essentially no welding, between said coating composition and the conductive tracks of said printed circuit board.

Said welding connection is usually located in a general way. Said coating is normally in single or multiple layers.

By polymer the inventors include polymers formed in situ from single or multiple monomers, linear, branched, grafted and crosslinked copolymers, oligomers, multipolymers, multimonomer polymers, polymer blends, grafted copolymers, polymer blends and alloys, as well as interpenetrating networks. of polymers. (RIP)

The thickness of the coating is usually from 1 nm to 2 pm, more usually from 1 nm to 500 nm, even more usually from 3 nm to 500 nm, even more usually from 10 nm to 500 nm and much more normally from 10 nm to 250 nm. The coating preferably has a thickness of 10 nm to 100 nm, in various gradients, 100 nm being a preferred thickness. In another embodiment, the thickness of the coating is 10 nm to 30 nm. However, the optimum thickness of the coating will depend on the properties required of the PCI. For example, if a very high environmental hardness is required (high resistance to corrosion and abrasion), a thicker coating can be used. Additionally, the thickness of the coating can be optimized with different thicknesses in different locations through the PCI, depending on which characteristic is being optimized (for example, protection of the environment against the conductivity of the Z axis). The thickness of the coating and the composition of the flux may vary to optimize the protection characteristics against the environment and provide particularly strong welding joints.

The halo-hydrocarbon coating may be continuous, substantially continuous (in particular on surfaces to be welded and surfaces not welded between them or adjacent to them and, more particularly, on substantially all exposed and vulnerable surfaces of the PCI) or not continued. For a very high level of protection against the environment, a substantially continuous coating may be required. However, a non-continuous coating may be sufficient for other purposes.

By "halo-hydrocarbon polymer" is meant a polymer with a straight or branched chain or ring carbon structure with 0, 1, 2 or 3 halogen atoms attached to each carbon atom in the structure. The halogen atoms could be the same halogens (for example, fluorine) or a mixture of halogens (for example, fluorine and chlorine). The term "halo-hydrocarbon polymer" as used herein includes polymers containing one or more unsaturated groups, such as double and triple carbon-carbon bonds, and the polymer containing one or more heteroatoms (atoms that are not of C, H or halogen), for example, N, S or O. Currently, the inventors prefer, however, that the polymer does not contain substantially any unsaturation (because unsaturation often results in reduced stability) and substantially no heteroatom of this type. Preferably, the polymer contains less than 5% heteroatoms as a proportion of the total number of atoms in the polymer. Preferably, the polymer contains less than 5% double or triple carbon-carbon bonds as a proportion of the total number of carbon-carbon bonds. The molecular weight of the polymer is preferably greater than 1000 amu.

The polymer chains may be straight or branched and there may be crosslinking between the polymer chains. The halogen can be fluorine, chlorine, bromine or iodine. Preferably, the polymer is a fluorohydrocarbon polymer, a chlorohydrocarbon polymer or a fluorochlorohydrocarbon polymer in which 0, 1, 2 or 3 fluorine or chlorine atoms are attached to each carbon atom in the chain .

Examples of preferred polymers include:

- PTFE, PTFE type material, fluorinated hydrocarbons, chlorinated fluorinated hydrocarbons, halogenated hydrocarbons, halo-hydrocarbons or copolymers, oligomers, multipolymers, multimonomer polymers, polymer blends, combinations, alloys, branched chain, grafted copolymers, crosslinked variants of these materials and also interpenetrating polymer networks (RIP).

- PCTFE (polychlorotrifluoroethylene) and copolymers, oligomers, multipolymers, multimonomer polymers, polymer blends, combinations, alloys, branched chains, grafted copolymers, crosslinked variants of this material and also interpenetrating polymer networks (RIP).

- EPCTFE (polychlorotrifluoroethylene ethylene copolymer) and copolymers, oligomers, multipolymers, multimonomic polymers, polymer blends, combinations, alloys, branched chains, grafted copolymers, crosslinked variants of this material and also interpenetrating polymer networks (RIP).

- Other fluoroplastics, including the following materials and copolymers, oligomers, multipolymers, multimonomic polymers, polymer blends, combinations, alloys, branched chains, grafted copolymers, crosslinked variants of these materials as well as interpenetrating polymer networks (RIP): ETFE ( copolymer of ethylene and tetrafluoroethylene), FEP (copolymer of tetrafluoroethylene and hexafluoropropylene), PFA (copolymer of tetrafluoroethylene and perfluorovinyl ether), PVDF (polymer of vinylidene fluoride), THV (copolymer of tetrafluoroethylene dichlorophenyl) PV-hephenophene (PV) copolymer of vinylidene fluoride and hexafluoropropylene), MFA (copolymer of tetrafluoroethylene and perfluoromethylvinyl ether), EFEP (copolymer of ethylene, tetrafluoroethylene and hexafluoropropylene), HTE (copolymer of hexafluoropropylene and ethylene fluoride and tetrafluoroethylene and tetrafluoroethylene chloride fluoroplastics

Most preferably, the polymer is a polytetrafluoroethylene (PTFE) type material and in particular polytetrafluoroethylene (PTFE).

Lower wettability can be achieved using a coating in which the halo-hydrocarbon is a highly branched polymer, a copolymer, a combination of polymers or a mixture of polymers.

It is desirable that the coating composition have one or more of the following properties, and preferably substantially all: the ability to deposit in the form of continuous films, without cracks, holes or defects; relatively low gas permeability that provides a significant barrier to gas permeation and prevents gaseous corrosion and oxidation through the coating; the ability to weld selectively without the need for prior removal and to achieve good welding joints comparable to other surface finishes currently available; the ability to withstand multiple thermal cycles; chemical resistance to corrosive gases, liquids and saline solutions, in particular environmental pollutants; that has low surface energy and 'wettability'; that is stable inert material at normal PCI temperatures; having good mechanical properties, including good adhesion to PCI materials and good resistance to mechanical abrasion; improved electrostatic protection; relatively low permeability to the liquid and saline solution, to prevent corrosion of the liquid "through" the coating; and, in general, that it is beneficial to the environment compared to existing processes when used in the present application.

The invention can also provide other electrical and / or electronic devices, or other items (such as pipes or other plumbing apparatus) to which welding connections are to be made, having such a coating. For example, the invention could be used to coat bare wires (especially copper wires) used in wire bonding techniques. Cable junction is a method of interconnection between an integrated circuit in the form of a bare die and the conductor frame within the integrated circuit or between the bare die and a PCI. The cable traditionally used has been gold or aluminum, but more recently there has been considerable interest in the use of copper cable for several reasons, including the significant differential cost with gold. In the union of cables, usually two methods of union are used, the wedge joint and the ball joint, both use different combinations of heat, pressure and ultrasonic energy to make a weld at one or both ends of the cable. To achieve a good bond, both the cable and the area to which it joins must be free of contaminants, including oxidation. It is a conventional practice to apply a gold finish to the bonding area to avoid oxidation. The coating of the present invention in a copper junction zone will also provide a surface without oxidation, allowing wire junction joints to be made using gold, aluminum or copper wire, either by wedge or ball joint, but at a cost significantly lower than the conventional gold finish in the bonding zone. When using copper wire, it is also beneficial to apply the halo-hydrocarbon coating to the wire to prevent oxidation after the wire has been manufactured and before storage. In addition, the halo-hydrocarbon coating provides additional protection against oxidation during the bonding process. Alternatively, in another embodiment of the invention, electrode electrodes could be coated. The polymer coating preferably provides a good barrier against permeation of liquids and Atmospheric gases, most importantly oxygen, which would normally react with the conductive tracks, usually copper tracks, to form a tarnish layer, usually copper oxide, on the surface of the track. As a result, the coated circuit board can be stored for long periods of time, up to several months or years, without damaging oxidation of the conductive tracks. Optical microscopy, scanning electron microscopy and back-scatter electron imaging have been used to investigate the nature, continuity and thickness of the coating. X-ray dispersive energy analysis has been used to map the levels and distribution of halogens in the coating. Measurements of surface activation and surface wettability using chemical solvent solutions provide an indication of the potential to act as a protective coating.

Once the manufacturer is ready to install components on the blank PCI, it is not necessary to clean the PCI or remove the coating layer before the welding process. This is because, surprisingly, the halo-hydrocarbon polymer used provides a coating that has the unusual property that it can be welded through to form a weld joint between the conductive track on the plate and the electrical component. Flux is generally required in this welding technique. At the end, a welding process that uses heat could only be used to selectively "remove" the coating, for example, laser welding. Additional alternatives are welding, laser-powered welding, ultrasonic welding or the use of conductive adhesives. Another possible technique is wave welding; This technique may require selective flux. The welding used can be lead welding or lead free welding. In general, there is no reduction in the strength of the weld joint as might be expected and, in fact, in certain circumstances, the weld joint may be stronger than a conventional weld joint. Furthermore, in certain circumstances, the present invention can prevent the formation of dendrites on the solder joints, in particular when lead-free soldering is used.

Therefore, the present invention provides an alternative technique for applying surface coatings of metals (such as tin, silver, nickel and gold) to the PCI conductive tracks to prevent oxidation of the conductive tracks before welding. The present invention has the advantage that it is based on a low-cost process, does not use toxic metals such as nickel, is environmentally friendly and is safer than current industrial metal plating processes. It also simplifies the PCI manufacturing process and is compatible with current industrial welding processes. In addition, it has the additional benefit of "weld through" properties, thus avoiding the need to remove the coating before welding.

An additional feature of the halo-hydrocarbon polymer coating is that it is only removed in the areas where welding and / or flux is applied. Therefore, in the areas of the PCI where the components are not fixed by selective welding, the coating remains intact, maintaining a protective layer on the plate and conductive tracks, which provides a barrier against corrosion by atmospheric gases such as Sulfur dioxide, hydrogen sulfide, nitrogen dioxide, hydrogen chloride, chlorine and water vapor and other corrosive materials, thus preventing corrosion by environmental pollutants. The halo-hydro-carbon polymer coating is also substantially impermeable to corrosive liquids and liquids. Consequently, it is possible to fix components to the circuit board in a series of stages with significant periods of time elapsing between each stage; This could provide a number of advantages to the manufacturer. This coating is not destroyed by the welding process in areas other than the selected welding areas, therefore, in the non-welded areas, the PCI can be reprocessed and / or reworked by welding at a later stage. In addition, once the PCI assembly is completed, the un soldered areas of the PCI remain coated with the halo-hydrocarbon polymer that forms a permanent barrier against environmental corrosion. There is no need for additional expensive overlay stages, such as conformal coating.

The conductive tracks on the printed circuit board may comprise any conductive material. The possible materials from which conductive tracks can be made are metals such as copper, silver, aluminum or tin or conductive polymers or conductive inks. The preferred material for the tracks is copper. Conductive polymers tend to absorb water and swell and, therefore, coating conductive polymers with a halo-hydrocarbon polymer layer can prevent water absorption.

Another feature of the coated PCI of the invention is that the impedance of the z axis is very low compared to the impedance on the x and y axes. By z axis is understood the axis that points to the plane of the PCI. The coating has a high impedance on the x and y axes, which demonstrates good insulation properties. However, the impedance is relatively low on the z axis. This allows electrical contact through the coating without having to remove it. This is particularly advantageous for applications such as keyboards, switch contacts, test points and the like. This characteristic can be further optimized by controlling the properties of the coating, for example, by controlling the thickness of the layer, its composition and the conditions of the process in the coating process and the nature of the coating process.

In summary, the invention prevents oxidation and / or other environmental damage, for example, modulation of thermal stability, scratching, corrosion and chemical resistance and the high barrier effect for the conductive tracks of the blank PCI and provides environmental protection of the assembled PCI, usually with a Initial stage of coating the blank PCI with a halo-hydrocarbon polymer.

The present specification also provides a method of protecting a printed circuit board to which a welding connection is to be made, a method comprising providing a printed circuit board, which is preferably blank, having a surface exposed to the environment and that it has no welding, or essentially no welding, on said surface exposed to the environment, and applying to that surface in a thickness of 1 nm to 10 pm of a composition comprising a halo-hydrocarbon polymer by a deposition technique Thin film, preferably by plasma deposition, chemical vapor deposition (DQV), molecular beam epitaxy (EHM), creation of interpenetrating polymer networks (RIP), monolayer surface absorption (ASM) of monomer polymers to form polymers in situ, polymer alloys or sputtering. Alternative deposition techniques are chemical vaporization of plasma-enhanced steam (DQV-PP), plasma deposition at high / atmospheric pressure, metallo-organic-chemical vapor deposition (DMOQV) and chemical vapor deposition of laser-enhanced steam (DQV-PL). Additional alternatives are liquid coating techniques, such as liquid immersion, spray coating, rotation coating and sol / gel techniques.

The method may depend on the coating thickness required. Liquid coating techniques may be preferred for thicker coatings, while plasma deposition techniques may be preferred for thinner coatings. The thickness of the coating is normally from 1 nm to 2 pm, more usually from 1 nm to 500 nm, even more usually from 3 nm to 500 nm, even more usually from 10 nm to 500 nm and much more normally from 10 nm to 250 nm. The coating preferably has a thickness of 10 nm to 100 nm, with 100 nm being a preferred thickness. The halo-hydrocarbon polymer is preferably a fluorohydrocarbon polymer, a chlorohydrocarbon polymer or a fluoro-chlorohydrocarbon polymer, which may also contain micropigments and small amounts of other performance additives (being a common practice in the industry of polymers) and can be, for example, polytetrafluoroethylene (PTFE) type materials. The method of applying the halo-hydrocarbon polymer to the blank PCI is plasma deposition, although all the other techniques mentioned above would also be applicable.

Plasma deposition techniques are widely used for the deposition of coatings in a wide range of industrial applications. The method is an effective way to deposit continuous thin film coatings using a dry and ecological technique. The PCI are coated in a vacuum chamber that generates a gaseous plasma comprising ionized gaseous ions, electrons, atoms and neutral species. In this method, the PCI is introduced into the vacuum chamber that is first pumped at pressures normally in the range of 10'3 to 10 mbar (10‘3 to 10 hPa). A gas is then introduced into the vacuum chamber to generate a stable gas plasma and then one or more precursor compounds are introduced into the plasma either as gas or liquid to allow the deposition process.

The precursor compounds are normally halogen-containing hydrocarbon materials, which are selected to provide the desired coating properties. When introduced into the gaseous plasma, the precursor compounds are also ionized / decomposed to generate a range of active species that will react on the surface of the PCI, usually through a polymerization process, to generate a thin halo-hydrocarbon coating. Preferred precursor compounds are perfluoroalkanes, perfluoroalkenes, perfluoroalkynes, fluoroalkanes, fluoroalkenes, fluoroalkynes, fluorochloroalkanes, fluorochloroalkenes, fluorochloroalkynes or any other fluorinated and / or chlorinated organic material (such as fluorohydrocarbons, chlorofluorocarbons) chlorofluorocarbons.

In another aspect of the invention, the surface of said printed circuit board has a coating of a metal fluoride with a thickness of 5 nm or less and a coating of a composition comprising one or more halo-hydrocarbon polymers with a thickness of 1 to 10 pm on the metal fluoride coating.

In one embodiment, the metal halide layer may be a monolayer, or substantially a monolayer, or a few monolayers, or comprise a metal halide zone of layers on the surface. A metal halide layer of this type can be very robust and inert and prevents the formation of oxide layers or other tariffs that prevent an effective welding. The metal halide layer can be formed when active species in the gas plasma react with the metal surface or can be enhanced using a higher concentration of fluorine species. Then, the halo-hydrocarbon layer can be deposited in combination with the metal halide layer. The two layers can be differentiated, axially or spatially or, alternatively, there may be a gradual transition from the metal halide to the halo-hydrocarbon. It is possible that the metal halide layer protects the metal from oxidation, while the halo-hydrocarbon layer provides environmental protection against corrosive gases and / or liquids, as well as protection against oxidation. In addition, if the coating wears out over time due to mechanical abrasion, the underlying metal fluoride layer will prevent the accumulation of oxidation, which will allow contact to still be made. The nature and composition of the plasma deposited coating depends on a number of conditions: the selected plasma gas; the precursor compound used; plasma pressure; the coating time; plasma power; the arrangement of the camera electrodes; the preparation of the incoming PCI; and the size and geometry of the camera. Normally, the plasma deposition technique can be used to deposit thin films of a monolayer (usually a few angstroms (Á)) at 10 micrometers (preferably 5 micrometers), depending on the previous settings and conditions. The plasma technique itself only affects the upper surface of the PCI and is generally fully compatible with the PCI itself, without causing damage or other unwanted effects. An advantage of plasma coating techniques is that the deposited coating accesses all surfaces of the PCI and, therefore, vertical surfaces will also be covered, such as those that are only accessible through the holes in the PCI and any outgoing If a particular area of the PCI should not be coated with polymer, for example, gold contacts at the edge of the PCI, then these areas can be masked during the plasma deposition process.

In a variant of the plasma process, it is possible to use the plasma method for in situ cleaning of the PCI surface before plasma deposition using an active gas plasma. In this variant, an active gas plasma is normally used in the same chamber for cleaning the PCI before the introduction of the precursor compound for the plasma deposition step. The active gas plasma is based on a stable gas, such as hydrogen, oxygen, nitrogen, argon, methane, ethane, other hydrocarbons, tetrafluoromethane (CF ⁴ ), hexafluoroethane (C ² F ⁶ ), tetrachloromethane (CCU), other hydrocarbons fluorinated or chlorinated, other rare gases or a mixture thereof. In a particular embodiment, the PCI could be cleaned with the same material to be deposited. For example, a fluorinated or chlorinated hydrocarbon such as tetrafluoromethane (CF ⁴ ) or hexafluoroethane (C ² F ⁶ ) or hexafluoropropylene (C ³ F ⁶ ) or octafluoropropane (C ³ F ⁸ ) could be used in the plasma method to clean the surface of the PCI and place a layer of a halo-hydrocarbon polymer and / or a layer of metal fluoride (or chloride).

The present invention relates to a method of making a solder connection to a printed circuit board cited in claim 1. A printed circuit board is disclosed to which a solder connection has been made in accordance with said method in claim 9.

The invention provides a method of making a connection to a printed circuit board coated with a composition comprising a halo-hydrocarbon polymer, a method comprising applying solder and flux to the printed circuit board at a temperature and during a time of so that the solder is bonded to the metal and the composition is dispersed and / or absorbed and / or vaporized and / or dissolved and / or reacted locally. The action of the flux and the increase in temperature alone will generally interact with the halohydrocarbon polymer to remove the coating locally from the area of the PCI to which the flux is applied. The temperature is normally from 200 ° C to 300 ° C, preferably from 240 ° C to 280 ° C and much more preferably from 260 ° C. In one embodiment, the halo-hydrocarbon polymer can be dissolved and / or absorbed by the flux. The inventors have discovered that there is often a balance between the required temperature and the acidity or other aggressiveness of the flux. Therefore, milder fluxes may be sufficient if higher temperatures are used and vice versa. In another embodiment, the self-melting action of copper fluoride on the copper surface and any decomposition of the polymer coating to release fluoride and / or HF to initiate the application of the flux (auto flux application) can be exploited. In the end, the inventors have discovered that, in certain cases, a method of realizing the welding connection can dispense with a flux if a sufficiently high temperature is used and, therefore, localized heating could be applied. Surprisingly, the composition is generally only specifically removed from the area where the solder and / or the flux is applied and, therefore, the composition remains fixed to the surface of the PCI to the solder joint. This provides protection against the advantageous environment of the conductive tracks of the PCI to the solder joint.

The flux used in the invention could be a resin / rosin flux, an organic flux, an inorganic flux, a halide-free flux, an unclean flux, a residue-free flux or a low-solids flux. A resin / rosin flux could be, for example, a synthetic resin or a natural rosin. An organic flux could be, for example, an organic acid such as lactic acid or an acrylic acid; an organic salt such as dimethylammonium chloride (DMA HCl); or an organic amine such as urea. An inorganic flux could be, for example: an inorganic salt such as zinc chloride, sodium chloride, potassium chloride or sodium fluoride; or an inorganic acid such as hydrochloric acid or nitric acid. An example of an unclean flux is a rosin flux. Other fluxes used more widely for industrial applications such as general welding, brazing and autogenous welding, or for cleaning or etching a metal surface (eg, borax) could also be used in the present invention. The flux used in this method is normally a soft flux, such as a "non-clean" flux that does not require a later stage of PCI cleaning. The flux may optionally be part of a solder paste. The choice of flux may depend on the nature of the coating, in particular the thickness and composition of the coating. A thicker and stronger coating may require the use of a more aggressive flux. A composition comprising the active ingredient or flux ingredients that remove the halo-hydrocarbon composition from the plate could also be used in the present invention.

It is also possible to use a composition comprising a halo-hydrocarbon polymer to protect from the surroundings a printed circuit board to which a solder connection is to be made through the composition, without its prior removal, by dispersion and / or absorption and / or vaporization of the composition optionally in the presence of a flux.

The environment may contain gaseous agents such as sulfur dioxide, hydrogen sulfide, nitrogen dioxide, hydrogen chloride, chlorine, ozone or water vapor, or liquids such as water, water in which the above corrosive gases dissolve, solutions salt or other spills. These gases are usually present in highly polluted environments, such as cities with air pollution problems. A particular environmental hazard against which the present invention protects PCI is the atmospheric humidity in which one or more of the corrosive gases listed above dissolve. The inventors have discovered that the invention is capable of protecting PCI against such hostile environments.

It is also possible to use a composition comprising a halo-hydrocarbon polymer to provide storage stability, preferably long-term storage stability, for a printed circuit board, which is preferably blank, to which it is preferably blank, to which has to be made a welding connection. As mentioned earlier, the conductive tracks in the PCI tend to oxidize if left exposed to the atmosphere. Oxidation reactions are normally the formation of metal oxides by reaction with atmospheric oxidation, but also include other oxidation reactions, for example, when metallic copper oxidizes to, for example, Cu + or Cu2 +. The composition avoids these oxidation reactions, so that a blank PCI can be stored for long periods of time, without oxidation of the conductive tracks. Therefore, after a long period of storage, good welding connections can be made to the PCI by conventional welding techniques, in the presence of flux, without pre-cleaning steps.

It is also possible to use a composition comprising a halo-hydrocarbon polymer to prevent oxidation and / or corrosion of the conductive tracks of a blank printed circuit board prior to welding application to said conductive tracks and / or the formation of a solder connection.

Description of the drawings

Figure 1 shows a welding profile of a reflux oven used with a commercial solder paste containing lead.

Figure 2 shows a welding profile of a reflux oven used with a commercial lead-free solder paste.

Figure 3 is an image of a PCI coated of the invention with a water droplet on the surface, demonstrating the low surface energy, low wettability, the liquid impervious nature of the surface coating.

Figure 4 is a cross-sectional image of a brazing joint made by welding through the coating on a PCI of the invention.

Figure 5 is a cross-sectional image of a strong solder joint formed on a PCI of the invention, demonstrating the formation of good quality copper-tin intermetallic products on the upper side of the lower copper surface.

Figure 6 is a MEB (Scanning Electron Microscopy) image of the edge of a 1 µm thick coating polymer on a PCI of the invention, shown at a magnification of x30,000. Figure 7 is an IER Image (backscatter electron image) showing an example area of a coated PCI of the invention, demonstrating the continuity of the coating in excess of 99.8% coverage.

Figure 8 is a MEB / XDE image showing a selectively removed coating region of a PCI of the invention by the action of the flux at a temperature, for a coating of 1 micrometer of nominal thickness. The image on the left shows where the flux has been applied selectively. The image on the right shows that the coating has been selectively removed in the area to which the flux was applied.

Figure 9 is an XDE spectrum showing the carbon / fluorine composition of the copper coating of a PCI of the invention.

Figure 10 is an image of legs of IC components plucked from a soldered PCI of the invention, showing strong solder joints. In strict tests, the joints eventually fail by fracturing the copper contact adapter to join the plate substrate, rather than at the solder joint.

Figure 11 is an image of welded contact adapters torn from the welded PCI of the invention, showing strong solder joints. In strict tests, the joints eventually fail by fracturing the copper contact adapter to join the plate substrate, rather than at the solder joint.

Figure 12 is a MEB image and an XDE image showing the presence of polymer coating to a weld joint edge formed on a PCI of the invention.

Figure 13 is an optical microscopy image showing a series of good quality solder joints formed on a PCI of the invention.

Figure 14a is an EFX spectrum of a set of thin coatings of the invention showing various contributions of C-F and Cu-F materials.

Figure 14b is an EFX spectrum showing CF-containing material for a thick coating. Examples

Preparation of coated printed circuit boards

Printed circuit boards were obtained from a manufacturer that had been engraved and cleaned, but to which the surface finish had not been applied. These plates were then treated by plasma deposition to generate the halogen-containing coating. The PCI was introduced into the vacuum chamber that was first pumped at pressures in the range of 10.3 to 10 mbar (10.3 to 10 hPa). A gas was then introduced into the vacuum chamber to generate a stable gas plasma and then a precursor hydrocarbon compound containing halogen was introduced into the plasma to allow the deposition process. When introduced into the gas plasma, the precursor compound also decomposed / ionized to generate a range of active species that reacted on the surface of the PCI to generate a thin halogen-containing coating. A series of experiments were performed on these treated plates.

Example 1

A commercial solder paste containing lead was applied by manual dosing of a syringe into several component adapters on one side of the PCI. Several integrated circuits were placed on the adapters that had solder paste on them. The PCI was then placed in a reflux oven where the welding profile had been configured as shown in Figure 1. Subsequently, the joints were visually examined with a microscope, where it was discovered that they had good wetting characteristics. Some of the joints were then separated by pulling the component with a tool. In each case, the integrated circuit leg was removed from the solder, leaving the connection to the PCI contact adapter intact.

Example 2

The above tests were repeated using a lead-free solder paste with a modified reflux profile as shown in Figure 2, with similar results.

Example 3

The flux was only applied to two PCI regions and heated to 260 ° C for ten seconds and five minutes. The test showed that the coating was no longer present in the areas where flux had been applied to any of the PCI. However, the coating remained intact in the areas where flux had not been applied.

Example 4

Shear strength test

Eight sets with four PCI finishes were prepared for the shear test. There were two assemblies for each PCI finish. Each assembly had seven 1206 chip resistors and four 0805 chip capacitors assembled. Fourteen resistors 1206 and eight 0805 capacitors of each assembly finish were subjected to shear test to determine the maximum shear strength (RMC) of the welding joints for each finishing assembly.

Test conditions

The plate was mounted in a shear test device. The separation height of the chisel tool on the surface of the PCI was 80 | jm and the width of the chisel tool was 2 | jm. During each test, the shear tool moved forward at a defined speed of 100 jm / s against the test component and the force was controlled until the welding joint fixation was broken. The shear test device used is the Dage 4000 Series, with a DS100 test head.

Results of initial shear strength tests

Example 5

The PCI surface energy table below shows greater hydrophobia with the coating process time:

Claims (9)

1. A method of making a solder connection to a printed circuit board, in which: A surface of the printed circuit board has a coating comprising one or more halo-hydrocarbon polymers with a thickness of 1 nm to 10 pm; Y the printed circuit has conductive tracks and there is no welding, or essentially no welding, between the coating and the conductive tracks, wherein said method comprises applying solder and flux to the coating without prior removal, at a temperature and for a time so that the solder adheres to the conductive tracks and the coating is dispersed and / or absorbed and / or vaporized locally. 2. The method according to claim 1, wherein the coating has a thickness of 10 nm to 100 nm. 3. The method according to claim 1 or 2, wherein the halo-hydrocarbon polymer is a fluorohydrocarbon. 4. The method according to any one of the preceding claims, wherein the halohydrocarbon polymer contains less than 5% heteroatoms as a proportion of the total number of atoms in the polymer, and preferably the polymer has a linear chain structure or branched and contains one or more heteroatoms selected from nitrogen, sulfur and oxygen. 5. The method according to any one of the preceding claims, wherein the surface of said printed circuit board has a coating of a metal fluoride with a thickness of 5 nm or less and a coating of a composition comprising one or more more halocarbon polymers in a thickness from 1 nm to 10 pm on the metal fluoride coating. 6. The method according to any one of the preceding claims, wherein the coating has different thicknesses in different locations. 7. The method according to any one of the preceding claims, wherein the conductive tracks comprise copper and the halo-hydrocarbon polymer is a PTFE type material. 8. The method according to any one of the preceding claims, further comprising the initial step of depositing the plasma deposition coating of one or more precursor compounds selected from: a perfluoroalkane; a perfluoroalkene; a perfluoroalkine; a fluoroalkane; a fluoroalkene; a fluoroalkine; a fluorochloroalkane; a fluorochloroalkene; and a fluorochloroalkine. 9. A printed circuit board to which a welding connection has been made, said printed circuit board being obtainable by a method as defined in any one of claims 1 to 8.